President Obama has said that chemical weapons would be a "red line" that would warrant U.S. intervention in Syria. U.N. independent commissioner Carla Del Ponte said yesterday that Syrian rebels have used sarin gas. Free Syrian Army leaders deny that.

In terms of human rights and all not being fair in love or war, understanding sarin may help a bit in framing discussions of morality as the U.S. considers putting more lives into the fray.

On April 22, 1915 in Belgium, the German army killed or injured 5,000 Allied soldiers by releasing 150 tons of chlorine gas. That is regarded as the first modern use of large-scale chemical warfare, though the concept goes back to snake-venom-tipped arrows in the Stone Age. By 1937, German chemist Gerhard Schrader had developed an insecticide that the Nazis soon realized was a more toxic agent than chlorine gas: sarin. They did not use it in World War II, though, reportedly because they understood its potential and feared retaliation in kind.

In 1988, around 5,000 Kurds died at Halahbja after Iraq used both sarin and sulfur mustard. Sarin further became a household name after the 1995 Tokyo subway attack in which the religious cult Aum Shinrykio used sarin to kill 12 people and harm thousands more.

Sarin is unique in potency but not in mechanism. There are other drugs, pesticides, and plants that work the same way. They are called cholinesterase inhibitors.

Our nerves talk to each other by releasing chemicals called neurotransmitters. The amount of a particular neurotransmitter helps determine whether a nerve fires or not. What so-called nerve agents do is alter those neurotransmitters. They kink the signaling between our nerves, telling them to do things they normally do, but with altered frequency.

After a neurotransmitter has done its job, delivered its message, an enzyme usually comes along and demolishes it. But nerve agents block those enzymes. The enzyme can't break down the neurotransmitter, so the neurotransmitter stays around and keeps giving its message. If that message was, say, to release a little water onto your eye because your eye was dry, now the repeated message becomes "make your eyes water uncontrollably."

Here is a drawing of that reaction, just like in organic chemistry class (still a requisite for all U.S. doctors). The big block is the enzyme (acetylcholinesterase). In the top image, it's working normally: breaking down the neurotransmitter (acetylcholine) into smaller parts. In the bottom images you can see how the "nerve agent" (sarin in our case) just kind of hangs out in the "esteric site," so then the enzyme cannot do its job.

The action seems pretty benign on paper. But as acetylcholine builds up in our bodies, we become extremely uncomfortable and die. We are killed by the accumulation of our own normal neurotransmitter telling our own nerves to do the normal things they normally do, just in excess. One could draw an analogy to cancer. In this case, though, neurotransmitters live and die on an order of milliseconds, so it happens in a flash.

Within seconds of exposure to sarin gas (or liquid, which evaporates easily), we start to notice the immediate effects of acetylcholine buildup. First, our smooth muscles and secretions go crazy. The nerves to those areas keep firing, keep telling them to go. The nose runs, the eyes cry, the mouth drools and vomits, and bowels and bladder evacuate themselves. It is not a dignified state.

Since sarin has no smell or taste, the person may very well have no idea what's going on. Their chest tightens, vision blurs. If the exposure was great enough, that can progress to convulsions, paralysis, and death within 1 to 10 minutes.

If the exposure was not enough to kill them, though, the person should recover pretty quickly and completely. It is not the sort of agent that leaves people blind and infertile and glowing green. The U.S. military also has a lotion that can be applied immediately after exposure, Reactive Skin Decontamination Lotion, to good effect, among other antidotes. Most people exposed to sarin do not die. A large exposure is not a death sentence.

This is all chilling. Chemical weapons have connotations of being unnatural and disturbing in ways beyond the mechanical brutality of guns, tanks, and bombs. Deploying sarin in a civilian setting or on massive scales can be catastrophic. In war, though, where there is already so much horrifying violence, should this weapon in itself necessarily be a red line that escalates international involvement?